. The chordates. Chordata. 118 Comparative Morphology of Chordates VENOSUS COMMON CARDINAL- VEIN. Fig. 323. A diagram of the primitive (fish) heart, as seen in a median longi- tudinal section. Anterior is to the right. The course of blood in the heart (indicated by arrows) takes the form of a letter S. (After Keith. Courtesy, Neal and Rand: "Chordate Anatomy," Philadelphia, The Blakiston Company.) which lie just under the notoehord. The arches may be cartilaginous or bony. They are, however, much more strongly developed than the neural arches of cyclostomes. The maximum number of pha
. The chordates. Chordata. 118 Comparative Morphology of Chordates VENOSUS COMMON CARDINAL- VEIN. Fig. 323. A diagram of the primitive (fish) heart, as seen in a median longi- tudinal section. Anterior is to the right. The course of blood in the heart (indicated by arrows) takes the form of a letter S. (After Keith. Courtesy, Neal and Rand: "Chordate Anatomy," Philadelphia, The Blakiston Company.) which lie just under the notoehord. The arches may be cartilaginous or bony. They are, however, much more strongly developed than the neural arches of cyclostomes. The maximum number of pharyngeal clefts occurs in a shark, Heptanchus (Fig. 329A), having a pair of spiracles and seven pairs of gill-clefts. Most adult fishes have no spiracles and only five pairs of gill-clefts. In some cases there are fewer than five. The highly charac- teristic visceral skeleton, cartilaginous or bony, is developed around the pharynx, consisting of arches alternating in position with the pharyngeal clefts (Fig. 120). Most fishes have a breathing valve which prevents exit of water from the mouth when the pharyngeal wall contracts to force water out via the gill-chambers. Just inside the oral aperture are a dorsal and a ventral inwardly projecting fold of the oral lining (Fig. 322). Pressure of water against these flaps from behind forces them forward so that they meet and occlude the oral passage. In most fishes the heart consists of two chambers, a receiving chamber, the auricle, and a pumping chamber, the ventricle (Fig. 323). In lungfishes there are two auricles, the right one receiving blood from the general circulation and the left receiving blood from the lungs. The two auricles open into a common single ventricle. The nervous organs of fishes are arranged on the same plan as in all other vertebrates. Of all organ-systems, none is more conservative. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - colo
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